FIG. 1 illustrates a prior art hydraulic actuator used for aircraft. The actuator 10 is hydraulically powered. Pressurized hydraulic fluid is provided from a source (not illustrated) on a high pressure line 12 and is returned to the hydraulic source on a low pressure line 14. A conventional direct drive valve (DDV) or electro-hydraulic servo valve (EHSV) 16 controls the application of pressurized hydraulic fluid on hydraulic lines 18 and 20 which permit bidirectional flow of hydraulic fluid. The DDV or DHSV 16 reverses the direction of fluid flow on hydraulic lines 18 and 20 to reverse the direction of motion of hydraulic motor 22 on output shaft 24. The shaft 24 drives a transmission 26 of any conventional design which has a pair of outputs for driving a first panel 28 and a second panel 30 each in reversible directions. As indicated at zero degrees the panels are in contact with each other under a condition of preload in which each panel is applying a force to the other panel during which the hydraulic motor 22 is maintained in a stall condition with the application of high pressure hydraulic fluid being applied thereto on one of the hydraulic lines 18 and 20.
The maintenance of the preload condition at zero degrees as illustrated on the panels 28 and 30 has disadvantages when the hydraulic motor 12 is maintained in a stall condition. This generates high hydraulic fluid leakages. As a result the stall condition creates heat in the hydraulic fluid which must be dissipated by the aircraft via a heat exchanger or other heat radiating structure. Any requirement for a heat exchanger is a penalty in weight to an aircraft. This leakage represents a power loss to the aircraft. As a result the leakage values for conventional hydraulic motors may be too high for particular applications in aircraft. The mere addition of a brake to maintain preload would introduce problems in obtaining a quick release in that control communications with the brake would introduce time delays.